Lyman-alpha observations of comet Hyakutake with SWAN on SOHO

Abstract

The SWAN instrument on board SOHO is a Lyman-α (Lα) photometer able to map the sky intensity with a resolution of 1°, and a capability of microstepping (0.1°). SWAN is primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background, but was in addition extensively used to map the Lα emission of several comets since its launch in December 1995. Here we report observations of comet C/1996 B2 (Hyakutake). Its Lα emission cloud extended over more than 60° while approaching the Earth at 0.102 AU. A comparison with a simple model allowed hydrogen and H 2O production rates to be derived, while the comet approached closer to the Sun from 1.12 AU to 0.53 AU distance to the Sun, pre-perihelion. The derived H 2O production rate was found in fair agreement with other derivations (IUE and ground-based in the IR and UV), validating the Lα method. The H 2O production by SWAN was related to several other measurements of minor constituents in order to derive new values of abundance of CO, HCN, H 2CO, CH 3OH and CH 3CN. Most important, the D H ratio in comet Hyakutake is now found at 3 × 10 −4, as in comet Halley, while a previous estimate based on a wrong H 2O number had indicated a value twice lower, with important cosmogonic consequences. The time evolution showed a fast surge on 21 March, coinciding with the time of fragmentation of the nucleus as detected 3 days later at Pic du Midi. This surge is also confirmed by the detailed comparison of H column densities (observed vs model) as a function of the distance to the nucleus, showing a larger ratio in the inner region (younger atoms) than in the outer region (older atoms) on 21 March, and then a progressive filling-in of the H envelope. After the surge, there was a plateau for 16 days around 1.8 × 10 29 H 2O mol s −1, and then an increase following approximately a R −2 law. This behavior is interpreted as the surge and plateau corresponding to the fragmentation and total disruption/evaporation of a fragment of the nucleus, of approximately 200 m. Finally, it is argued that the first detection of ethane C 2H 6 in this comet (IR observations) might have been the result of the special circumstances (a large fragment disrupted very near the Earth) rather than revealing a new special class of ethane-rich comets as argued by other authors.